PRESIDENTIAL ADDRESS. 417 



ranges, these undulations remain perpetuated as domes and dimplinga of the 

 crust, which are aheady worn down or infilled respectively by denudation and 

 deposition. Their present forms and places record the last movements of the 

 earth-storm, just as a buckled tramway-rail records the passage of an earth- 

 quake. How shall we gauge to-day the intensity of their rise and fall? 



In the case of the city devastated by an earthquake, the debris is cleared 

 away, and our descendants in time discover the distorted rails beneath the 

 healing mantle of new grass. Will they realise from this alone the preliminary 

 tremors, the sudden arrival of the culminating vibration, the shock that over- 

 came the elasticity of the crust beneath them, and then the gradual establish- 

 ment of the conditions under which they have passed their peaceful lives '! 

 The crumpled wreckage lies there in evidence before them ; but how will they 

 distinguish the work of a few stormy seconds from that due to the gentle 

 earth-creep of a century ? 



Possible Breaks in the Slow Continuiiy of Earth-movciiitnt. 



2. Regions of Subsidence. 



It was probably E. Suess who brought home to most of us the importance of 

 regions of subsidence in defining the lowlands and the sea-basins from the up- 

 standing masses of the crust. While one region may be folded, another may be 

 broken into blocks ; and the two types of movement, that due to tangential 

 thrusting and that due to vertical uplift and down-faulting, may appear in the 

 same region and may alike play their part in producing a lowering of large 

 areas. The domes and dimples that occur beyond the region of acute crumpling 

 may be intensified into fault-blocks by fracture of their boundaries. W. Salomon ''* 

 has, moreover, shown us how the movements in the Rhine-trough, a typical region 

 of block-foundering, may be linked with those associated with over-thrusting. 

 The rise of one region is associated with the lowering of another. If catastrophes 

 are possible during uplift, we may look for them also during subsidence. 



The occurrence of lines of volcanoes in and along the edges of subsiding 

 areas may be regarded as evidence of the squeezing of a previously fused sub- 

 stratum, or as evidence of sub-crustal melting which has primarily caused the 

 subsidence. In either case, it is difficult to believe that the cracks along which 

 the cones become established are formed gently and without elements of surprise. 

 The dykes and cones represent a healing process. The accumulation of lava- 

 .sheets and the outpouring of ash frona localised centres obscure what has gone 

 on before. The uplift of the Pacific coast has included rapid stages, as 

 C. Darwin recognised,^' and as we have recently realised in Alaska. It is 

 improbable that the downward movements of the sea-Hoor adjacent to it have 

 been of a minor order, or that the larger movements of elevation have not been 

 accompanied by similar movements of collapse. 



The cutting-up of mountain-chains by transverse fractures has resulted in 

 the loss of huge blocks beneath the sea. In such cases it is clear that faulting 

 has run a long way ahead of denudation. The breaking of the chain that united 

 Andalusia and the western Alps, the falling in of the Tyrrhenian earth-block, 

 the subsidences at two separate epochs of the southern and northern Adriatic, 

 and the conversion of the hilly mass between the Balkans and Asia Minor into 

 the ^gean, set with islands, suggest a return to the girdling waters of the 

 Tethys. The spread of the Atlantic northward, by fracture across the tough 

 blocks of Armorican land, has led Pierre Termier to revive the story of 

 Atlantis *' as an episode of human times. Botanists and zoologists require a 

 recent Atlantic land-surface as a field for migration and a refuge during Glacial 

 cold. Who has recorded, except. the Egyptian priest of Sais, the precise mode 

 of its disappearance beneath the waves ? 



All trough-valleys, which are often called, somewhat misleadingly, rift- 

 valleys, raise the same questions as to the nature of the steps by which they 

 have been produced. The Rhine Vale, one of the most closely studied examples, 



" ' Die Bedeutung der Messen von Harnischen,' Zeitschr. deutsch geol 

 GesHl, vol. Ixiii. (1911), p. 515. 



*' Geol. Observations on S. America, Minerva edition, p. 293. 

 " ' L'Atlantide," Bvll. Institut ocianographique, No. 266 (1913). 

 1915. E B 



